Scientific investigation during the last two decades has substantiated a prominent role for brain amyloid-β (Aβ) in the pathogenesis of Alzheimer's disease. Existing evidence supports the contention that Alzheimer's disease (AD) is initiated by an excess level of Aβ in the brain. The neurotoxicity of Aβ leads to the death of neurons, inflammation of the brain, dementia, and AD (Selkoe, 1999; Selkoe and Schenk, 2003). Since Aβ occupies such a central role in AD pathogenesis, the reduction of Aβ in the brain has become a major therapeutic strategy for AD.
Aβ is a 40- or 42-residue peptide generated from the degradation of β-amyloid precursor protein (APP), a membrane protein, by two proteases known as γ-secretase and β-secretase (also known as memapsin 2, the name recommended by IUBMB's Enzyme Nomenclature Commission, or BACE1). In this pathway, memapsin 2 initiates the APP cleavage; then the second cleavage by γ-secretase, a multi-protein complex, produces Aβ. An excess level of the neurotoxic Aβ in the brain over a long period of time leads to the death of neurons, brain inflammation and other harmful events that mark the progression of Alzheimer's disease (AD). Thus, these proteases are major therapeutic targets.
The molecular entity of γ-secretase has not yet been conclusively identified although it is clear that this activity is associated with a membrane protein complex consisting presenilin-1, nicastrin and others (Wolfe, 2002). β-secretase was cloned and identified as a membrane anchored aspartic protease called memapsin 2 (Lin et al., 2000; U.S. Pat. No. 6,545,127; PCT Publication No. WO 2007/021886). Four other laboratories independently discovered this enzyme and assigned different names for this protease: BACE (Vassar et al., 1999) and ASP-2 (Yan et al., 1999); Hussain et al, 1999). Memapsin 2β-secretase (also referred to as Memapsin 2, β-secretase, and/or BACE1) is a class I membrane protein that includes a protease domain highly homologous to pepsin, a transmembrane domain and a cytosolic domain. The protease is synthesized in vivo with an N-terminal pro-region, which is cleaved by furin to remove a 33-residue pro-segment en route to the cell surface (Capell et al., 2000). The crystal structure of memapsin 2 protease domain (Hong et al., 2000) shows that it contains an extended active-site cleft characteristic of aspartic proteases, and that the Aβ-hairpin flap covers over the active-site cleft. As in other aspartic proteases, the flap must open to permit the entering of substrate into the active-site cleft.
Memapsin 2 initiates cleavage of amyloid precursor protein (APP) leading to the production of Aβ and the onset of AD. The native APP is a poor substrate of memapsin 2 (Lin et al., 2000; Ermolieff et al., 2000). The so-called “Swedish mutation” of APP at the P2-P1 subsites from Lys-Met to Asn-Leu enhances the hydrolytic efficiency by about 60 fold, increases Aβ production and manifests an early onset form of AD. The specificity of all eight substrate residues has been determined (Turner et al., 2001). Native memapsin 2 is glycosylated by three N-linked oligosaccharides. The hinge which links the catalytic unit to the transmembrane region is only 6 residues (Hong et al., 2000). The transmembrane domain contains three cysteines which are covalently linked to palmitic acids. This is consistent with the lipid raft localization of memapsin 2 in the membranes. The intracellular domain contains a signal for endocytosis from cell surface to endosomes, which likely involve the recognition of proteins such as GGA for transport through the clathrin-coated vesicles (He et al., 2002). The optimal pH for memapsin 2 activity is about 4.5.
At present, there is no disease-modifying therapy for the clinical treatment of AD that has received regulatory approval. The few available drugs for treating AD are mostly acetylcholinesterase inhibitors, such as Donepezil (Aricept), which can only mildly improve cognitive performance and treats the symptoms of AD rather than the root cause of the disease. Therefore, there is an acute need for the development of new treatments for this disease.
Due to the current limitations of AD treatment regimes, there remains a significant interest in and need for additional or alternative therapies for treating, stabilizing, preventing, and/or delaying AD. The present invention fulfills this and other needs.
The specification is most thoroughly understood in light of the references cited herein. Each of these references is hereby incorporated by the reference in its entirety.